[mythtv] fullscreen/window mode

[Wally Wallace]

I am a bit confused about the difference between fullscreen and windowed
viewing. I run X at 640x480 on my frontend and I would think that there
would be no difference between fullscreen and windowed on a 640x480
recording, but when I hit F I get a square window about
280x280(estimate). Is the window scaled down - I should probably look in
the source myself and figure it out but I thought I would ask here, I
hope nobody minds.
The reason I ask is I am trying to troubleshoot my frontend. 
I have very high cpu usage and I can't figure out why. My cpu is a
celeron 1.2 on a sis630 chipset. Mythtv is using xv. I have dri and drm
enabled. The network is 100mb full-duplex. I can play divx or mpeg's
through mplayer with only %25 cpu usage. When I run myth to watch a
recording or livetv recorded at 640x480 myth uses about 70% cpu in 4 or
5 processes and X uses 20-30% the rest. It is unwatchable because
the audio skips every other second and the video drops maybe 1/5
frames. 480x480 is barely watchable, but cpu usage is still near
100%. This just seems wrong to me, shouldn't my system handle
mythfrontend better? What can I do to continue troubleshooting?
Thanks